Disentangling Composite Spectrum Hot Subdwarfs

We give preliminary results from a spectroscopic study of composite spectrum hot subdwarfs (sd+late-type). We obtained spectra of a sample of hot subdwarfs selected from the Catalogue of Spectroscopically Identified Hot Subdwarfs (Kilkenny, Heber, & Drilling 1988) on the basis of near-infrared pho-tometry from the Two Micron All Sky Survey (2MASS). The sample consists of 20 photometric and spectroscopic single and 54 composite hot subdwarfs, 6 resolved (or barely resolved) visual doubles, and 5 objects with emission lines or broad absorption lines with emission cores. Spectra of 84 " standard " (single late-type) stars with Hipparcos parallaxes were also obtained for calibration. These observations cover 4600–8900Å with ∼3Å resolution. We measured equivalent width-like indices around Mg I b, Na I D, the Ca II infrared triplet, Hα, and Hβ. Using the single late-type star observations combined with model energy distributions, we explore how the measured indices of a composite spectrum vary as the temperature and luminosity of the late-type companion are varied and as the temperature and radius of the hot subdwarf are varied. We use the measured indices of the composite systems to estimate the temperature and gravity of the late-type star, taking into account the dilution of its spectral features by light from the hot subdwarf. Hot subdwarfs were selected from the Catalogue of Spectroscopically Identified Hot Subdwarfs (Kilkenny et al. 1988). The observed sample consists of 20 single and 54 composite hot subdwarfs (primarily classified as sdB) that have V ∼ < 13.5. Composite subdwarfs for this sample were defined to have J − K S ∼ > +0.05 (Stark & Wade 2003). There are also 6 subdwarfs that have close (<10 ′′) resolved companions, and 5 that show emission lines (4 met our composite color criterion). Eighty-four stars (52 main sequence, 32 subgiant) with parallax measurements from Hipparcos were selected to provide empirical calibration. They cover the Pop I main sequence from early F to early M-type, and post-main sequence subgiants from F to K-type (2.7 ∼ < V ∼ < 10.0, −1.1 ∼ < M V ∼ < 9.2, and 0.35 ∼ < B −V ∼ < 1.6). Spectra were taken with the GoldCam spectrograph on the Kitt Peak National Observatory 2.1m telescope. The wavelength interval 4600–8900Å was covered at ∼1.3 ˚ A/pixel, using two spectrograph setups.